Excitons in insulating cuprates

We study the electronic excitations near the charge-transfer gap in insulating CuO₂ planes, starting from a six-band model which includes p_π and dxy orbitals and Cu-O nearest-neighbor repulsion Upd. While the low-lying electronic excitations in the doped system are well described by a modified t-J model, the excitonic states of the insulator include hybrid dxy-p_π states of A2g symmetry. We also obtain excitons of symmetries B1g and Eᵤ, and eventually A1g, which can be explained within a one-band model. The results agree with observed optical absorption and Raman excitations.